When digital computers are used to measure or control the operation of equipment, processes or systems, it is often desirable or necessary to provide the computer with means to sense and measure analog inputs (such as voltages or currents which may vary over given ranges) in addition to discrete or digital inputs (such as from switches or pushbuttons). For example, if a digital computer, is to be used to monitor or control a chemical process, it must be able to measure temperatures, pressures and/or other process variables.
The usual way of doing this is to employ one or more analog-to-digital converters which allow the digital computer to measure analog voltages. The analog-to-digital converter(s) measures signals from various transducer circuits which convert temperatures, pressures and/or other quantities into analog voltages. In general, the increasing use of digital computers for measurement, computation and control makes it desirable to be able to convert a wide variety of physical quantities into digital values.
Given either a digital-to-analog converter or an analog-to-digital converter, it is possible to construct the complementary type of converter using appropriate feedback circuits. It is a common practice to construct medium to high accuracy, medium to high speed analog-to-digital converters by using a parallel type digital-to-analog converter and feedback control circuits. Such converters are sometimes called comparison converters. At the present time, the comparison type of analog-to-digital converter is the most widely used type for an input device for digital computer systems. Hence, it will be discussed in greater detail below.
In a comparison type converter, a medium to high accuracy, medium to high speed analog-to-digital converter is constructed by using a digital-to-analog converter and feedback control circuits. The digital-to-analog converter generates an analog voltage corresponding to a number in a digital register. Feedback control circuits adjust the number in the digital register until the converted analog voltage corresponds to an input analog voltage. A common practice is to perform a series of comparisons and adjustments of a binary digital number, one bit at a time beginning with the most significant bit, so as to achieve a relatively short conversion time. Converters using this technique are often referred to as successive approximation converters. For example, a 12-bit successive approximation converter, with a precision of one part in 4096 (2.sup.12), will require only twelve comparisons for a complete analog-to-digital conversion.
In conventional comparison type analog-to-digital converters, the accuracy and resolution are normally quite closely tied together. That is, in order to provide a high resolution, it is also necessary to provide a similarly high accuracy. In some applications, it may be desirable to provide a high resolution without necessarily providing a correspondingly high absolute accuracy. For example, suppose one wishes to weigh several objects to determine their uniformity of weight. One may wish to measure small differences with an accuracy of (say) a fraction of an ounce, without necessarily wanting to know the total weight of each object to a similar accuracy. Similarly, one may wish to measure the temperature of a chemical reaction vessel. It may be desirable to measure a rate of change of temperature by measuring differences of (say) a few hundredths of a degree over a short period of time, while it is not necessary to measure the temperature itself with a similar accuracy. In such applications, the ability to measure small differences or changes with a high resolution, but without necessarily a similarly high absolute accuracy, can be of value.
A critical element in the comparison type analog-to-digital converter is the digital-to-analog converter. A common practice is to use a parallel resistor network driven by a number of voltage or current switches, with each switch being controlled by one bit of the binary number to be converted. The resistor network is adjusted so that the relative weight given to the output of each voltage or current switch is one-half the weight given to the output of the switch corresponding to the next more significant bit.
Using an eight-bit binary converter as an example, the problem in the parallel digital-to-analog converter (and hence, the comparison type analog-to-digital converter) is that the voltage developed by the converter in response to, for example, a digital input of 10000000 (128 in decimal) may not be exactly one increment larger than the voltage developed for a digital input of 01111111 (127 in decimal). If the effective weighting values of the resistor network and/or the inaccuracies of the voltage or current switches are such that the effective weight given to the most significant bit is slightly low, the voltage generated for the conversion of 10000000 may actually be less than the voltage generated for the conversion of 01111111.
Adjustments are usually provided to set the weights given to some number of the most significant bits to overcome this problem. Unfortunately, resistor networks tend to drift with variations in temperature and with time, and the characteristics of semiconductor voltage or current switches may also drift, so that readjustment may be required from time to time. At the present time, 15 or 16 bits is about the practical limit for parallel resistor network type digital-to-analog converters and, hence, comparison type analog-to-digital converters.
In general, a comparison type analog-to-digital converter can provide a high resolution only by providing a correspondingly high absolute accuracy. Simply extending the number of bits in a comparison type converter by adding additional circuits is generally of no utility, as the values of the additional bits in the converted digital numbers will have no particularly significant relationship to the value of the analog input signal.
As an example, the resolution and accuracy situation in regard to conventional comparison type analog-to-digital converters is similar to the use of a balance and a set of weights of 16 ounces, 8 ounces, 4 ounces, 2 ounces and 1 ounce for weighing. Measuring a change of weight from 15 ounces to 16 ounces requires going from using the combination of the 8, 4, 2 and 1 ounce weights to using the 16 ounce weight. The accuracy of the 16 ounce weight must generally he better than the desired resolution. In contrast, a simple spring type scale may have an absolute error of several ounces when weighing an object of approximately 16 ounces. However, it is still capable of indicating a change of, say, one ounce and thus providing a usable resolution which is significantly finer than its absolute accuracy. Conventional analog-to-digital converters are, in general, not capable of providing useful resolutions finer than their absolute accuracies.
A further limitation of conventional comparison type analog-to-digital converters is that the conversion time can be quite slow for high resolution conversions. The successive approximation type converter requires a comparison for each bit of the digital output value, which must be made with an accuracy somewhat greater than the desired resolution. After each adjustment of the digital data value, it is necessary to wait until the digital-to-analog converter output has settled to a stable value and the comparison circuitry has measured the perhaps quite small difference between the digital-to-analog converter output voltage and the analog input voltage.
In the case of a high resolution analog-to-digital converter, this total process may be somewhat slow. At the present time, moderate accuracy successive approximation analog-to-digital converters (say up to eight bits) can be readily built to provide a complete analog-to-digital conversion in less than 10 microseconds. Commercially available high accuracy successive approximation converters (say 15 or 16 bits) typically require several hundred microseconds for a complete conversion. High speed analog-to-digital converters, using approaches other than the successive approximation technique described above, can perform complete conversions in a small fraction of a microsecond. However, these high speed converter designs are generally more expensive and are usually capable of providing only low to moderate accuracy conversions
Examples of the prior art related to the subject invention include U.S. Pat. No. 3,068,456 issued Dec. 11, 1962 to S. G. Nevius; U.S. Pat. No. 3,353,175 issued Nov. 14, 1967 to J. E. Brook, et al; U.S. Pat. No. 3,624,641 issued Nov. 30, 1971 to L. E. Brennan; and U.S. Pat. No. 3,745,559 issued July 10, 1973 to J. Mattern.